1. Field of the Invention
The invention relates to optical fiber lasers for use in communication networks and other laser applications where high laser power and good beam quality is required such as for example laser ablation and drilling as well as directed energy military applications.
2. Background of the Invention
Coherent beam combining of fiber lasers is an important technique toward developing compact high-power lasers with high brightness. One area of interest has been the recent emergence of fibers with multiple active cores. An optical fiber with a single core can either support a single transverse mode or multiple transverse modes to propagate along their axis and is, therefore, either called single mode or multimode fiber. In fact, single transverse mode fiber supports two modes with orthogonal polarization that have the same propagation constant in a symmetric (for instance circular symmetry) fiber. On the other hand, different transverse modes in multimode fiber have in general different propagation constants and other properties, e.g., different diffraction angles when the beam leaves the fiber.
Modern fiber fabrication techniques also allow incorporating multiple cores into the same cladding. Individual cores can either be single mode or multimode. These cores can either be interacting with each other (typically when the modes of individual cores overlap specially) or for all practical reasons non-interacting where light launched into an individual core will not be coupled into other cores during propagation. A multicore fiber (MCF) generally has a larger emitting area compared with a monocore fiber; meanwhile, as multiple emitters are distributed in an array and separated by passive regions, the thermal and stress problems encountered at high-power levels can be alleviated. However, if each emitter in the core array oscillates independently with random phase relationship, the output beam will diverge as fast as an individual emitter does. The low-brightness output beams from the incoherently combined core arrays will not be very beneficial for practical applications.
To coherently combine individual emitters and obtain a high-brightness output beam from the array, the relative phase between adjacent emitters should be locked, for example as demonstrated in early semiconductor laser arrays. For a typical MCF, the core array is typically either distributed in a ring, or distributed in a densely packed two-dimensional isometric pattern. In both scenarios, each single-mode core can evanescently couple with its neighboring cores, and different supermodes are formed and characterized by a fixed (locked) phase difference between adjacent emitters. Each supermode has its own distinctive intensity distribution and diffraction property, but only the fundamental in-phase supermode, i.e., all cores locked in the same phase, has a Gaussian-like far-field intensity distribution with an intensified central lobe of low divergence.
However, since mode competition exists inevitably in MCF laser cavities, it is equally important to build a fiber a laser cavity that establishes solely the in-phase mode and suppresses all higher-order modes.
To coherently combine the emissions of a core array into a phase-locked supermode, it is essential to develop a selective feedback mechanism that supports only one specific supermode with maximal gain and minimal loss while it discriminates all other modes with less gain and higher loss during cavity round trip. There currently exist a number of different mode selection techniques including Talbot-cavities, Fourier transform resonators, structured mirrors, and diffractive optics approaches to phase-lock multiple active cores (and also other multi-element gain structures such as doped waveguides or a number of singe-core fiber amplifiers) into exclusive fundamental in-phase mode operation. See FIGS. 4(a)-(b). However, all previously known techniques and laser cavity designs involve bulk optical components and free-space optics.
For example, among recent approaches to provide such a differential feedback, one scheme is to utilize the Talbot effect, which has been demonstrated earlier with diode, microchip, and CO2 waveguide laser arrays. Talbot cavity MCF lasers have also been reported recently with phase-locked high-brightness output beams obtained. However, in these MCF lasers, free-space optics, i.e., air gaps and bulk optical components, has become an inseparable part.
Further, the presence of free-space optics in a fiber laser cavity is practically undesirable, because it not only substantially expands the device size from a single piece of fiber to a bulky open-space setup but also introduces more cavity loss. In addition, the free-space optics, i.e. air gaps in the laser cavity and bulk optical components, not only cause alignment difficulties but also instabilities during high power laser operation. These serious stability issues can occur at high-power laser operation, e.g., thermal or environmental disturbances can easily affect the crucial and delicate cavity alignment. Because the unavoidable thermal changes of the optical components with increasing power will deteriorate the alignment and seriously affect the device performance, not to mention the decrease of laser efficiency due to additional cavity losses. It is clearly favorable to achieve an all-fiber aligning-free solution to phase lock multiple active cores for fiber laser devices in high-power operation. Therefore, it is a strong preference to eliminate any free-space optics and force the supermode selection to occur inside a confined waveguide, ideally, within an optical fiber. This will result in a truly all-fiber phase-locked MCF laser, which is free of optical alignment and robust against external disturbances.
In earlier all-fiber approaches to phase lock the emissions of a core array, the out-of-phase supermode has been selected by either an annular waveguide or a fiber mirror, while the in-phase supermode amplification has been achieved with a pulsed Gaussian beam.
Thus, while multicore fiber lasers that operate in a phase-locked status have great potential to provide high-power output beams with excellent beam quality and almost unlimited power scalability, current mode selection techniques, including the Talbot-cavities, Fourier transform resonators, structured mirrors, and/or diffractive optics approaches (to phase lock the multiple active cores into the in-phase supermode), all involve bulk optical components and free-space optics.
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